1. Field of the Invention
The present invention relates to an apparatus for manufacturing a semiconductor device having a circuit structured with a thin film transistor. For example, the invention relates to an apparatus for manufacturing an electro-optical device, typically a liquid crystal display device, and the structure of electric equipment mounted with such an electro-optical device as a component. Note that throughout this specification, the semiconductor device indicates general devices that may function by use of semiconductor characteristics, and that the above electro-optical device and electric equipment are categorized as the semiconductor device.
2. Description of the Related Art
In recent years, the technique of crystallizing and improving the crystallinity of an amorphous semiconductor film or a crystalline semiconductor film (a semiconductor film having a crystallinity which is polycrystalline or microcrystalline, but is not a single crystal), in other words, a non-single crystal semiconductor film, formed on an insulating substrate such as a glass substrate by laser annealing, has been widely researched. Silicon film is often used as the above semiconductor film.
Comparing a glass substrate with a quartz substrate, which is often used conventionally, the glass substrate has advantages of low-cost and great workability, and can be easily formed into a large surface area substrate. This is why the above research is performed. Also, the reason for preferably using a laser for crystallization resides in that the melting point of a glass substrate is low. The laser is capable of imparting high energy only to the non-single crystalline film without causing much change in the temperature of the substrate.
The crystalline silicon film is formed from many crystal grains. Therefore, it is called a polycrystalline silicon film or a polycrystalline semiconductor film. A crystalline silicon film formed by laser annealing has high mobility. Accordingly, it is actively used in, for example, monolithic type liquid crystal electro-optical devices where thin film transistors (TFTs) are formed using this crystalline silicon film and used as TFTs for driving pixels and driving circuits formed on one glass substrate.
Furthermore, a method of performing laser annealing is one in which a laser beam emitted from a pulse oscillation type excimer laser, which is large in output, is processed by an optical system so that the laser beam thereof becomes a linear shape that is 10 cm long or greater or a square spot that is several cm square at an irradiated surface to thereby scan the laser beam (or relatively move the irradiation position of the laser beam to the irradiated surface). Because this method is high in productivity and industrially excellent, it is being preferably employed. A laser beam that has been linearized into a laser beam that is 10 cm long or greater at the irradiated surface is referred as a linear laser beam throughout the present specification.
Different from when using a spot shape (square) laser beam which requires a front, back, left, and right scan, when using the linear laser beam, in particular, the entire irradiated surface can be irradiated by the laser beam which requires only scanning at a right angle direction to the linear direction of the linear laser beam, resulting in the attainment of a high productivity. To scan in a direction at a right angle to the linear direction is the most effective scanning direction. Because a high productivity can be obtained, using the laser beam that is emitted from the pulse oscillation type excimer laser and processing it into a linear laser beam by an appropriate optical system for laser annealing at present is becoming a mainstream.
Shown in FIGS. 1A and 1B is an example of the structure of an optical system for linearizing the sectional shape of a laser beam on the irradiated surface. This structure is a very general one and all aforementioned optical systems conform to the structure of the optical system shown in FIGS. 1A to 1B. This structure of the optical system not only transforms the sectional shape of the laser beam into a linear shape, but also homogenizes the energy of the laser beam in the irradiated surface at the same time. Generally, an optical system that homogenizes the energy of a beam is referred to as a beam homogenizer.
If the excimer laser, which is ultraviolet light, is used as the light source, then the core of the above-mentioned optical system may be made of, for example, entirely quartz. The reason for using quartz resides in that a high transmittance can be obtained. Further, it is appropriate to use a coating in which a 99% or more transmittance can be obtained with respect to a wavelength of the excimer laser that is used.
The side view of FIG. 1A will be explained first. Laser beam emitted from a laser oscillator 101 is split at a right angle direction to the advancing direction of the laser beam by cylindrical lens arrays 102a and 102b. The right angle direction is referred to as a longitudinal direction throughout the present specification. When a mirror is placed along the optical system, the laser beams in the longitudinal direction will curve in the direction of light curved by the mirror. These laser beams in this structure are split into 4 beams. The split laser beams are then converged into 1 beam by a cylindrical lens 104. Then, the converged laser beam are split again and reflected at a mirror 107. Thereafter, the split laser beams are again converged into 1 beam at an irradiated surface 109 by a doublet cylindrical lens 108. A doublet cylindrical lens is a lens that is constructed of 2 pieces of cylindrical lenses. Thus, the energy in the width direction of the linear laser beam is homogenized and the length of the width direction of the linear laser beam is also determined.
The top view of FIG. 1B will be explained next. Laser beam emitted from the laser oscillator 101 is split at a right angle direction to the advancing direction of the laser beam and at a right angle direction to the longitudinal direction by a cylindrical lens array 103. The right angle direction is called a vertical direction throughout the present specification. When a mirror is placed along the optical system, the laser beams in the vertical direction will curve in the direction of light curved by the mirror. The laser beams in this structure is split into 7 beams. Thereafter, the split laser beams are converged into 1 beam at the irradiated surface 109 by the cylindrical lens 104. Thus, homogenization of the energy in the longitudinal direction of the linear laser beam is made and the length of the longitudinal direction is also determined.
The above lenses in the optical system are made of synthetic quartz for correspondence to excimer laser. Furthermore, coating is implemented on the surfaces of the lenses so that the excimer laser will be well transmitted. Therefore, the transmittance of excimer laser through one lens is 99% or more.
By irradiating the linear laser beam linearized by the above structure of the optical system in an overlapping manner with a gradual shift in the width direction thereof, laser annealing is implemented to the entire surface of a non-single crystal silicon film to thereby crystallize the non-single crystal silicon film and thus its crystallinity can be enhanced.
A typical method of manufacturing a semiconductor film that is to become the object to be irradiated is shown next.
First, a 5 inch square Corning 1737 substrate having a thickness of 0.7 mm is prepared as the substrate. Then a 200 nm-thick SiO2 film (silicon oxide film) is formed on the substrate and a 50 nm-thick amorphous silicon film (hereinafter denoted by xe2x80x9ca-Si filmxe2x80x9d) is formed on the surface of the SiO2 film. Both films are formed by employing the plasma CVD apparatus.
The substrate is exposed under an atmosphere containing nitrogen gas at a temperature of 500xc2x0 C. for 1 hour to thereby reduce the hydrogen concentration in the film. Accordingly, the laser resistance in the film is remarkably improved.
The XeCl excimer laser L3308 (wavelength: 308 nm, pulse width: 30 ns) manufactured by Lambda Co. is used as the laser apparatus. This laser apparatus generates a pulse oscillation laser and has the ability to output an energy of 500 mJ/pulse. The size of the laser beam at the exit of the laser beam is 10xc3x9730 mm (both half-width). Throughout the present specification, the exit of the laser beam is defined as the perpendicular plane in the advancing direction of the laser beam immediately after the laser beam is emitted from the laser irradiation apparatus.
The shape of the laser beam generated by the excimer laser is generally rectangular and is expressed by an aspect ratio which falls under the range of the order of 2 to 5. The intensity of the laser beam grows stronger towards the center of the beam and indicates the Gaussian distribution. The size of the laser beam processed by the optical system having the structure shown in FIG. 1 is transformed into a 125 mmxc3x970.4 mm linear laser beam having a uniform energy distribution.
Based upon an experiment conducted by the present inventor, when irradiating a laser to the above-mentioned semiconductor film, the most suitable overlapping pitch is approximately {fraction (1/10)} of the width (half-width) of the linear laser beam. The uniformity of the crystallinity in the film is thus improved. In the above example, the half-width of the linear laser beam was 0.4 mm, and therefore the pulse frequency of the excimer laser was set to 30 hertz and the scanning speed was set to 1.0 mm/s to thereby irradiate the laser beam. At this point, the energy density in the irradiated surface of the laser beam was set to 420 mJ/cm2. The method described so far is a very general method employed for crystallizing a semiconductor film by using a linear laser beam.
When an extremely attentive observation is made to a silicon film that has been laser annealed by using the above-mentioned linear laser beam, very faint interference patterns were seen in the film. The cause of the interference patterns seen in the film resides in that the laser beam is split and assembled in 1 region, and therefore the split light brings about interference with each other. However, because the coherent length of the excimer laser is about several microns to several tenths of microns, a strong interference will not occur. As a result, the influence imparted by the above-mentioned interference to a semiconductor device is extremely small.
The excimer laser is large in output and capable of oscillating pulses repetitively at a high frequency (approximately 300 hertz under the present situation), and hence is often used in performing crystallization of a semiconductor film. In recent years, advances have been made in manufacturing low temperature poly-silicon TFTs used in liquid crystal displays. Accordingly, the excimer laser is employed in the crystallization process of semiconductor films.
Further, the largest output of an YAG laser is remarkably improved. Because the YAG laser is a solid state laser, it is easier to handle and maintain compared with the excimer laser which is a gas laser. Therefore, in the crystallization process of a semiconductor film, if the YAG laser is substituted for the excimer laser, an astounding improvement in cost performance can be expected. On the basis of the background such as the above, the present applicant is making an examination in the possibility of using the YAG laser in the crystallization process of a semiconductor film.
It is known that the YAG laser outputs a laser beam having a wavelength of 1065 nm as the fundamental wave. The absorption coefficient of this laser beam with respect to silicon films is extremely low, and therefore the laser beam as it is cannot be used in the crystallization process of the a-Si film, which is one of the silicon films. However, the laser beam, i.e., the fundamental wave, can be modulated into having a shorter wavelength by using a non-linear optical crystal. Due to the modulated wavelengths, the laser beam is named a second harmonic (wavelength 533 nm), a third harmonic (wavelength 355 nm), a fourth harmonic (wavelength 266 nm), and a fifth harmonic (wavelength 213 nm).
Since the wavelength of the second harmonic is 533 nm, it has sufficient absorption to an a-Si film that is about 50 nm thick, and hence can be used in crystallizing the a-Si film. In addition, the third harmonic, the fourth harmonic, and the fifth harmonic also have a high absorption to the above-mentioned a-Si film, and therefore similar crystallization can be performed.
The largest output of the second harmonic from the current general-purpose YAG laser is about 1500 mJ/pulse. Further, the largest output of the third harmonic thereof is about 750 mJ/pulse and the largest output of the fourth harmonic thereof is about 200 mJ/pulse. The largest output of the fifth harmonic is further lower than the aforementioned largest outputs, and thus if the fifth harmonic is used in crystallizing the a-Si film, mass production will become extremely worse. Taking into consideration both the output of the laser beam and its absorption to the a-Si film, at the present level, it is best to use the second harmonic and the third harmonic.
In the case of using the YAG laser to crystallize the semiconductor film, nonetheless, the shape of the laser beam at the irradiated surface is preferably linear for mass production. Hereinbelow, an examination is made on the possibility of applying the above-mentioned optical system to the YAG laser without any modifications made thereto.
First, the difference between the beam shape of the YAG laser and the beam shape of the excimer laser will be described. The shape of the laser beam emitted from the excimer laser is generally rectangular, whereas the shape of the laser beam emitted from the YAG laser is both circular and rectangular. To process a rectangular laser beam into a linear laser beam is comparatively easy since transformation is made from a rectangular shape to a rectangular shape. However, to transform a circular beam into a rectangular linear laser beam is comparatively difficult. Therefore, judging from the shape of the beam only, it is better to use the rectangular beam emitted from the YAG laser than to use the circular beam emitted therefrom.
Hereinafter, a description will be made on the uniformity of energy between the beam of the YAG laser which emits the rectangular beam and the beam of the YAG laser which emits the circular beam, and an examination is conducted to discern which one of the YAG lasers is suitable as a substitute for the excimer laser.
The type of YAG laser which has a beam shape that is circular irradiates a strong light (flash lamp and laser diode, hereinafter denoted by LD) for exciting a cylindrical crystal rod, thereby obtaining laser oscillation. On the other hand, the type of YAG laser which has a beam shape that is rectangular irradiates a strong light to a parallelepiped crystal rod that structures a system called a zigzag slub, thereby obtaining laser oscillation.
Comparing the energy uniformity of the beam oscillated from the YAG laser which has a beam shape that is circular with that of the excimer laser, the energy uniformity of the former is in general not good. This non-uniformity of energy originates from a temperature distribution of the above-mentioned cylindrical crystal rod which occurs from the application of a strong light. As can be readily surmised from the shape of the aforementioned cylindrical crystal rode, the temperature of the temperature distribution thereof becomes lower towards the exterior of the cylinder. Thus, a function similar to that of a lens is added to the aforementioned cylindrical crystal rod, thereby worsening the energy uniformity of the beam. This phenomenon is generally called the thermal lens effect.
A system conceived for the purpose of restraining the above thermal lens effect is a zigzag slub system of the YAG laser. The structure of the zigzag slub system of the YAG laser will be briefly explained with reference to FIG. 2 in the following.
The rod system YAG laser obtains laser oscillation by excitation of the cylindrical crystal called a crystal rod. However, in the case of the zigzag slub system YAG laser, the shape of the crystal rod is parallelepiped. A parallelepiped crystal 202 is irradiated by excitation lamps 203a and 203b, for example, an LD and a flash lamp, to thereby obtain laser oscillation. Electric power is supplied to the excitation lamps 203a and 203b from a power source 208. Furthermore, the parallelepiped crystal 202 is cooled by a cooler 207.
Arranging resonant mirrors 201 and 204 diagonally to the parallelepiped crystal 202 is a characteristic of the zigzag slub system. The resonant mirrors 201 and 204 are arranged parallelly in a state facing each other and sandwiching the parallelepiped crystal 202. Each of the surfaces of the parallelepiped crystal 202 and the resonant mirrors have no parallel positional relationship. By appropriately adjusting the positional relationship, light reflected from the resonant mirrors will advance in a zigzag way within the parallelepiped crystal. When laser is oscillated at this point in this state, a large amount of light will exit from a side surface of the parallelepiped crystal resulting in a large lost of energy, and thus becoming unusable. In order to prevent this drawback, reflector mirrors 205 and 206 are arranged at the side surfaces of the parallelepiped crystal to thereby prevent light escaping from the parallelepiped crystal 202. Gold-plated mirrors, for example, may be used as the reflector mirrors 205 and 206.
By adopting the above structure, the laser beam not only passes through the interior portion of the parallelepiped crystal rod, but also passes through the exterior portion thereof. Thus, influence to the biased laser beam of the temperature distribution of the crystal is less than the case of using the cylindrical crystal rod. Consequently, influence from the thermal lens effect becomes lesser thereby enhancing the uniformity of the beam.
Thus, it can be determined from the above examination that the zigzag system YAG laser is suitable as a substitute for the excimer laser than the type of YAG laser which employs the cylindrical crystal rod because the shape of the laser beam emitted from the zigzag system YAG laser is similar to that of the excimer laser and the uniformity of the beam thereof is higher.
Next, consideration is made in regards to a difference in the coherent length of the YAG laser and the excimer laser. As stated above, the coherent length of the excimer laser is about several microns to several tenths of microns, and therefore the occurrence of light interference when the laser beam passes through the aforementioned optical system, which splits and then converges the laser beam into 1 beam again, is thus very weak. On the other hand, the coherent length of the YAG laser is quite long, about 1 cm or more. Hence, the influences of the interference due to this long coherent length of the YAG laser cannot be ignored.
If the laser beam emitted from the YAG laser is passed through the optical system shown in FIG. 1 to be processed into a linear laser beam, then a linear laser beam 300 having an energy distribution with repetitive strong and weak regions in a lattice pattern as shown in FIG. 3A is formed.
The lattice pattern energy distribution is caused by the light interference. In FIG. 3A, darker lines 301 denotes regions where the energy is comparatively high and blank lines 302 between the darker lines 301 denotes regions where the energy is comparatively low.
Using the linear laser beam 300, which has a lattice pattern energy distribution, to crystallize the a-Si film will nonetheless cause non-uniform crystallization in the surface of the a-Si film. Shown in FIG. 3B is the appearance of a front surface of a silicon film 303 crystallized by the linear laser beam. As described in the above, the linear laser beam is irradiated on the a-Si film in the width direction of the linear laser beam and overlaps each other in a manner that is about {fraction (1/10)} of the length of the width of the laser beam. Therefore, the stripes parallel to the linear direction of the linear laser beam eliminate each other out, becoming inconspicuous. However, lines 304 and 305 that are parallel to the width direction of the laser beam strongly remain. In FIG. 3B, darker lines 304 denotes regions where the energy is comparatively high and blank lines 305 between the darker lines 304 denotes regions where the energy is comparatively low.
The present invention has been made in view of the above, and therefore has an object thereof to select an appropriate YAG laser as a substitute for an excimer laser used for the crystallization of a semiconductor film, and to resolve the aforementioned problem of an interference pattern, thereby providing a laser irradiation apparatus for attaining a polycrystalline silicon film having very little stripe patterns.
The inventors of the present invention have selected a zigzag slub system YAG laser, which has a rectangular beam shape, as the appropriate YAG laser for employment in the crystallization of a semiconductor film. In the present invention, it is important that the shape of the beam is rectangular, and there is no particular problem in using a YAG laser of a different system. However, the present inventor considers the zigzag slub system as the most suitable system among the current systems of the YAG laser at present. Further, the laser irradiation apparatus disclosed in the present specification is not particularly limited to one that emits a rectangular laser beam, but a laser irradiation apparatus that emits a circular laser beam may also be used.
As a problem that occurs when the YAG laser is used in crystallizing the semiconductor film is that interference, as mentioned in the above paragraph, is liable to occur in the YAG laser compared with the excimer laser.
The present invention will provide a technique to suppress an influence of the interference pattern. As mentioned before, the oscillation wavelength of the YAG laser includes a fundamental wave (1.06 um), a second harmonic (0.53 um), a third harmonic (0.35 um), a fourth harmonic, a fifth harmonic, and so forth.
Shown in FIG. 4A is a schematic view of a simplified beam homogenizer having the second harmonic of the YAG laser as a light source. A fundamental wave emitted from a light source 401 is converted into the second harmonic by a non-linear optical element 402. Because components of the fundamental wave still remain in the laser beam converted into the second harmonic, at a beam splitter 403, only the fundamental wave is transmitted and the second harmonic is reflected. Next, the light path of the second harmonic is bend 90 degrees by a mirror 404, and then split into 2 beams by a cylindrical lens array 405. Thereafter, the split beams are converged into 1 beam at an irradiated surface 407 by a cylindrical lens 406. At the irradiated surface 407 at this point, lights having equivalent wavelengths advance in opposite directions to each other, thereby interfering with each other. The pattern of interference that has developed in the irradiated surface 407 is shown in FIG. 4B. The pattern of interference illustrated in FIG. 4B is one in which plural patterns of wave shapes that change with time are overlapped. Throughout the present specification, a plural number of patterns of interference are shown, but all will be shown in the same method as that of FIG. 4B.
A stationary wave is formed when lights of equivalent wavelengths advance in opposite direction from each other. However, in a portion where energy is weak, the energy thereof becomes extremely weak. Thus, when a region having an immense energy difference is formed, a massive decline in the uniformity of crystallization by using laser is therefore unavoidable.
Accordingly, by utilizing the characteristic of the YAG laser capable of simultaneously emitting plural kinds of wavelengths of light, the present inventor designed a method of making the interference pattern inconspicuous by compositing YAG lasers of different wavelengths.
An example of a system that is capable of making the interference pattern inconspicuous is shown in FIG. 5A. Light (fundamental wave) oscillated from a resonator 501 of the YAG laser is converted into the second and third harmonic, besides being converted to the fundamental wave, via a non-linear crystal 502 for converting wavelengths. The fundamental wave is split by a beam splitter 503 which is provided with functions to satisfactorily penetrate the wavelength region of the fundamental wave, and to satisfactorily reflect the other wavelengths. Lights having the second and third harmonic reflected from the beam splitter 503 intermingled can thus be formed. Then at a beam splitter 504, only the second harmonic is reflected while the third harmonic is transmitted. Finally, the advancing direction of the third harmonic is alternated by a reflector mirror 505 so that the advancing direction thereof is the same as the advancing direction of the second harmonic.
Thus, a YAG laser capable of simultaneously emitting 3 types of lights, that is, the fundamental wave, the second harmonic, and the third harmonic, can be made through the above structure. Not much of the fundamental wave is absorbed by the silicon film, and hence is not used in the crystallization of the silicon film. The second and third harmonic are used in the crystallization thereof.
Shown in FIG. 34 is a wavelength dependence of a ratio of the absorption of light to a 55 nm-thick a-Si film formed on a glass substrate. As can be known from the graph, when light having a wavelength of 600 nm or less is used, there is a 10 percent or more absorption to the silicon film. Therefore, when the present invention is applied to a 55 nm-thick a-Si film, light having a wavelength of 600 nm or less is used.
The second harmonic is split into 2 beams by a cylindrical lens array 506. On the other hand, the third harmonic is split into 2 beams by a cylindrical lens array 507. The cylindrical lens arrays 506 and 507 are set at positions with equivalent focal lengths. Finally, a cylindrical lens 508 is arranged and the laser beam which has been split into 4 beams are composited to 1 region by the cylindrical lens 508.
The second harmonic and the third harmonic enter the cylindrical lens 508. Therefore, though there is a few percentage of difference in the focal length to the second harmonic and the third harmonic with each other, there is no influence to the present experiment. Quartz, which has a high transmissivity to both the second harmonic and the third harmonic, is used as the material of the lenses. Interference occurs in an irradiated surface 509 due to the fact that the second harmonic and the third harmonic advance in opposite directions from each other. A simulation result of the pattern of interference formed in the irradiated surface 509 is shown in FIG. 5B. It is apparent from the graph of FIG. 5B that nodes caused by the interference have disappeared.
FIG. 6 is a view illustrating the state of an interference that has occurred when the second harmonic and the third harmonic having equivalent swinging widths with each other are made to advance in opposite directions from each other at equivalent speeds. In FIG. 6, the longitudinal axis denotes the intensity of light and the lateral axis denotes a position. As can be understood from FIG. 6, it is apparent that the energy distribution of light is more balanced in this case than in the case of using only the second harmonic.
An output of the second harmonic emitted from the YAG laser having the structure of FIG. 5 is about twice the size of an output of the third harmonic. Therefore, in order to synthesize the second harmonic and the third harmonic emitted from the YAG laser having the structure of FIG. 5 to thereby make the interference patterns inconspicuous, it is necessary to consider synthesizing a second harmonic that has a swinging width of {square root over (2)} and a third harmonic that has a swinging width of 1. The simulation result thereof is shown in FIG. 7. Making the swinging width of the second harmonic of FIG. 6 to {square root over (2)} times is the result of FIG. 7. Similar to the result of FIG. 6, the energy distribution of light is also satisfactorily balanced in FIG. 7.
Thus, from the above results, it can be surmised that the contrast of a strong and weak pattern of the energy caused by the interference can be easily suppressed by synthesizing lights having different wavelengths from each other. Actually, similar results can be expected from synthesizing the second harmonic and the fourth harmonic and synthesizing the third harmonic and the fourth harmonic. An example of synthesizing the second harmonic and the fourth harmonic is shown in FIG. 8, and an example of synthesizing the third harmonic and the fourth harmonic is shown in FIG. 9. It is apparent that the energy is made uniform in both examples. Energy uniformity can also be achieved even if 3 types or more different wavelengths of laser beams are mixed together. In other words, by irradiating each of the laser beams having different wavelengths from each other to the same region and at the same time, uniformity of the laser beam in the same region can be improved.
The above-mentioned method of making the interference pattern inconspicuous by synthesizing laser beams of different wavelengths is applied to the optical system for forming linear laser beams that is illustrated in FIG. 1. The interference pattern is made inconspicuous by making laser beams of different wavelengths advance in opposite directions from each other at equivalent speeds. For example, the effect of making the interference pattern inconspicuous can be obtained in an optical system for forming linear laser beams illustrated in FIG. 10. The optical system shown in FIG. 10 is a beam homogenizer, similar to the optical system shown in FIG. 1. The basic perspective of constructing the lenses in both optical systems is the same.
The role of the optical system structured as shown in FIG. 10 will be explained. The aforementioned YAG laser is used as a laser oscillator. The YAG laser oscillates the second harmonic and the third harmonic in addition to the fundamental wave. The fundamental wave outputted from a laser resonator 1001 is converted into the second harmonic and the third harmonic by a non-linear optical element 1002. Components of the fundamental wave remain in the second harmonic and the third harmonic. Next, the fundamental wave is separated by a beam splitter 1003 while the second harmonic and the third harmonic are introduced to a beam splitter 1004. The second harmonic and the third harmonic are further separated into a second harmonic and a third harmonic by the beam splitter 1004.
Penetrating the beam splitter 1004, the advancing direction of the third harmonic is bent by reflector mirrors 1005 and 1006. As a result, the second harmonic and the third harmonic exit at oblique opposite angle positions in the form of being parallel with each other.
The second harmonic is first split into 2 beams in the vertical direction by a cylindrical lens array 10071, and then split into 2 beams in the horizontal direction by a cylindrical lens array 10081. These split laser beams are converged into 1 beam at an irradiated surface 1011 by cylindrical lenses 10091 and 10101.
On the other hand, the third harmonic is first split into 2 beams in the vertical direction by cylindrical lens array 10072, and then split into 2 beams in the horizontal direction by a cylindrical lens array 10082. These split beams are converged into 1 beam at the irradiated surface 1011 by cylindrical lenses 10092 and 10102.
The reason why the structure of FIG. 10 is able to make the interference pattern become inconspicuous will be explained with reference to FIG. 11. FIG. 11 is a top view of the optical system of FIG. 10. The laser beam split into 4 laser beams and converged into 1 beam in the longitudinal direction of the linear laser beam are denoted by the following names, respectively: laser beam A (the laser beam that passes the outermost right-hand side), laser beam B (the laser beam that passes the inner side of the right-hand side), laser beam C (the laser beam that passes the inner side of the left-hand side), and laser D (the laser beam that passes the outermost left-hand side).
Laser beam A is the third harmonic and laser beam D is the second harmonic. Both laser beams are advancing in opposite directions from each other at equivalent speeds, and therefore the effect of making the interference pattern in the irradiated surface 1011 inconspicuous is attained. Laser beam B is the third harmonic and laser beam C is the second harmonic. Both laser beams are advancing in opposite directions from each other at equivalent speeds, thereby attaining the effect of making the interference pattern in the irradiated surface 1011 inconspicuous. That is, laser beam A and laser beam D erase an interference effect with each other. Furthermore, laser beam B and laser beam C erase an interference effect with each other.
Shown in FIG. 12A is the state of an interference in the irradiated surface 1011, which is calculated by a computer, when the laser beams in FIG. 11 are all second harmonic. In this graph, it is apparent that the formation of loops and nodes are distinctive due to the influence of interference. On the other hand, shown in FIG. 12B is the state of an interference in the irradiated surface 1011 when the above method is adopted in the optical system of FIG. 11. The loops and nodes that were seen in FIG. 12A have disappeared, and hence it is discerned that the energy has been made uniform. The essence of the present invention is in making each of the lights having different wavelengths uniform and then synthesizing the respective uniform lights into 1 light in the irradiated surface.
Accordingly, the possible stripe pattern developing in the semiconductor film when the semiconductor film is laser annealed with a YAG laser that has been processed into a linear laser beam can thus be made unnoticeable. Although cited in the present specification is an example of taking out laser beams having different wavelengths from each other from 1 laser oscillator, there is no influence of any kind inflicted upon the essence of the present invention even if laser beams having different wavelengths from each other are taken out from 2 laser oscillators. In this case, a trigger is tuned in to so that the emission of lasers having different wavelengths from each other may be performed at the same time. The present invention is not limited to the YAG laser but can be applied to all laser irradiation apparatuses having a long coherent length such as a glass laser and an Ar laser. In addition, the present invention is not limited to a linear laser beam that has a linear section in the irradiated surface but is also applicable to a rectangular laser beam having a small aspect ratio. The present invention is further applicable to a square shape laser beam.
That is, according to the present invention, there is provided a laser irradiation apparatus that irradiates a laser beam with a section which becomes linear, square-like, or rectangular in an irradiated surface, characterized by comprising a laser oscillator that emits a plurality of laser beams having different wavelengths from each other, an optical system for processing the plurality of sectional laser beams having different wavelengths from each other into a square-like or rectangular laser beam in the irradiated surface, respectively, and making an energy distribution uniform, and a stage for arranging an object to be irradiated.
According to another aspect of the present invention, there is provided a laser irradiation apparatus that irradiates a laser beam with a section which becomes linear in an irradiated surface, characterized by comprising a laser oscillator that emits a plurality of laser beams having different wavelengths from each other, an optical system for processing the plurality of sectional laser beams having different wavelengths from each other into a linear laser beam in the irradiated surface, respectively, and making an energy distribution uniform, and a means of relatively moving the object to be irradiated to the laser beam.
In any of the above-mentioned inventions, since the laser oscillator is a YAG laser and the maintenance of the laser apparatus is easy to manage, productivity is increased and is thus preferable. Further, the YAG laser is capable of generating harmonics readily, and hence is suitable for use in the present invention.
Also, in any of the above-mentioned inventions when the object to be irradiated is a non-single crystal silicon film, laser beams having a wavelength of 600 nm or less may be used as the laser beams having different wavelengths from each other because the processing efficiency is high. For example, it is good to use the combination the second harmonic and the third harmonic of the YAG laser, or the combination the second harmonic and the fourth harmonic of the YAG laser, or the combination the third harmonic and the fourth harmonic of the YAG laser because the processing efficiency is high. Other than the YAG laser, a YVO4 laser, a glass laser, etc. can be used in the present invention.
Both of the above-mentioned laser apparatuses have a load/unload chamber, a transfer chamber, a pre-heat chamber, a laser irradiation chamber, and a cooling chamber. Both laser apparatuses are preferred for they can be used in mass production.
Further, according to another aspect of the present invention, there is provided a laser irradiation method that simultaneously irradiates each of a plurality of laser beams having different wavelengths from each other to a same region, characterized in that the shape of the laser beam in the same region is square-like or rectangular.
Further, according to another aspect of the present invention, there is provided a laser irradiation method that simultaneously irradiates each of a plurality of laser beams having different wavelengths from each other to a same region, characterized in that the shape of the laser beam in the same region is linear.
Further, according to another aspect of the present invention, there is provided a laser irradiation method that simultaneously irradiates each of a plurality of laser beams having different wavelengths from each other to a same region of a substrate having a non-single crystal semiconductor film formed thereon, characterized in that the shape of the laser beam in the same region is square-like or rectangular.
Further, according to another aspect of the present invention, there is provided a laser irradiation method that simultaneously irradiates each of a plurality of laser beams having different wavelengths from each other to a same region of a substrate having a non-single crystal semiconductor film formed thereon, characterized in that the shape of the laser beam in the same region is linear and that the linear laser beam is irradiated to the non-single crystal semiconductor film while relatively scanning the linear laser beam to the non-single crystal semiconductor film.
Still further, according to another aspect of the present invention, there is provided a method of manufacturing a semiconductor device provided with a TFT formed on a substrate, characterized by comprising the steps of forming a non-single crystal semiconductor film on the substrate and simultaneously irradiating a plurality of laser beams having different wavelengths from each other to a certain region of the non-single crystal semiconductor film.
Still further, according to another aspect of the present invention, there is provided a method of manufacturing a semiconductor device provided with a TFT formed on a substrate, characterized by comprising the steps of forming a non-single crystal semiconductor film on the substrate and simultaneously irradiating a plurality of laser beams having different wavelengths from each other to a region of the non-single crystal semiconductor film to thereby transform the non-single crystal semiconductor film into a crystalline semiconductor film.
In any one of the above-mentioned inventions, the laser beam is a YAG laser and the maintenance of the laser apparatus can be easily managed, and therefore the inventions thereof are appreciated. Also, because the plurality of laser beams having different wavelengths from each other have a wavelength of 600 nm or less in any one of the above-mentioned inventions, their absorption to the semiconductor film is large and hence the inventions thereof are appreciated. As laser beams having a wavelength of 600 nm or less, there are, for example, the second harmonic, the third harmonic, and the fourth harmonic of the YAG laser.